Add extension methods for each LRO type to OperationExtensions

[annelo-msft]

We currently have a single extension method for retrieving the result directly from an LRO, without having to use more complicated LRO patterns:

public static ValueTask<Response<IReadOnlyList<RecognizedReceipt>>> WaitForCompletionAsync(this Task<RecognizeReceiptsOperation> operation, CancellationToken cancellationToken = default);

See this thread for discussion.

We will add one such extension method for each of the LRO methods in both FormRecognizerClient and FormTrainingClient.

[maririos]

See file https://github.com/annelo-msft/azure-sdk-for-net/blob/db94c229fd9da844a674ee2b69633454fbf0ed56/sdk/formrecognizer/Azure.AI.FormRecognizer/src/OperationExtensions.cs in PR #11048

[AlexanderSher]

Can we extract a base interface, implement it in every operation, and have only one extension method?

[AlexanderSher]

Something like:

public static async ValueTask<Response<T>> WaitForCompletionAsync<T>(this Task<Operation<T>> operation, CancellationToken cancellationToken = default)
{
    Operation<T> o = await operation.ConfigureAwait(false);
    return await o.WaitForCompletionAsync(cancellationToken).ConfigureAwait(false);
}

public static async ValueTask<Response<T>> WaitForCompletionAsync<T>(this Task<Operation<T>> operation, TimeSpan pollingInterval, CancellationToken cancellationToken = default)
{
    Operation<T> o = await operation.ConfigureAwait(false);
    return await o.WaitForCompletionAsync(pollingInterval, cancellationToken).ConfigureAwait(false);
}

[annelo-msft]

Per discussion with @mayurid, addressed for Preview 1 with #11444 and moving to Preview 3 for final implementation.